Today's energy shortages make it increasingly necessary to store energy which becomes available during periods of relatively low energy demand for use during peak demand periods. For example, solar energy is readily available during relatively low day time demand periods but is frequently unavailable during the peak evening hour demand periods.
It has been suggested to store excess energy with inertial energy storage wheels or rotors. Such energy storage requires that excess energy, normally electrical power, is used to drive an electrical motor-generator to spin the rotor at often very high rates of rotation. To recover the energy, the motor-generator is operated in its generator mode to generate electricity while correspondingly decreasing the rotor's rate of rotation, thereby converting the rotors inertial energy into electrical power. To store a meaningful amount of energy, the rotors have to be spun at rates as high as 20,000 rpm and more, depending on the diameter of the rotor. This severely stresses the rotor and requires that it be specially constructed so that it can withstand the centrifugal forces generated by such high rates of rotation.
It is known that the stress to which a rotating ring is subjected comprises both hoop or circumferential stresses, which subject the ring material to tension, and radial stresses which subject the material to translaminar tension. In the radial direction the tensile stresses are carried by the matrix material only which is relatively weak. Since the radial tensile stress depends upon the ring thickness-to-radius ratio the ring must be relatively thin to maintain the stresses within the limits of the matrix material.
To achieve the required high energy storage densities materials with a high strength-to-weight ratio necessary. The materials with the highest strength-to-weight ratios currently are fiber materials such as those used for the reinforcement of plastic composites. The fiber composites, therefore, offer the potential of very high energy storage densities. Problems exist, however, due to the orthotropic properties of the composites. They possess very high strength in the direction of the fibers, that is, in a circumferential direction, and very little strength in the transverse directions, that is, in a radial direction. Thus, fiber composite materials can withstand only very limited radial forces.
A theoretical ring with no radial thickness would not be subjected to any radial stress but to hoop stresses only. Thus, to limit the radial stresses in such rings to acceptable values, their radial thickness must be relatively small. Accordingly, it has been suggested to construct inertial energy storage rotors by combining a plurality of relatively thin, concentric rings into one storage rotor. The rings are mounted to a concentric hub, which in turn rotates about a vertical axis. The rings are interconnected by resilient, e.g. elastomeric spacers disposed between each adjacent inner and outer ring. U.S. Pat. Nos. 3,683,216 and 3,741,034 generally describe the construction of inertial energy storage wheels constructed of a plurality of concentric rotor rings carried by a common hub. Elastomeric spacer rings connect each inner ring to its adjacent outer ring.
The spacer rings are constructed of an elastomeric material so that the rings can move relative to each other in a radial direction; that is, during rotation of the hub each outer ring expands or dilates a greater amount than its adjacent inner ring because the rate of expansion of each ring due to centrifugal forces is directly related to its mean diameter. Consequently, as the rate of rotation of the wheel increases the gaps between adjacent rings also increase. The spacer rings must accommodate this differential expansion of the rotor rings. Additionally, the spacer rings function to center the rings with respect to each other.
Under the high centrifugal forces, and the resulting large radial dilations to which rotors of the type disclosed in the above-referenced U.S. patents are subjected the use of elastomeric spacer rings between each pair of rotor rings has drawbacks. The differential radial expansion of the rings is relatively large. This severely stresses the elastomeric spacer ring. More seriously, it places a severe stress on the connection, e.g. the bond between the spacer ring and the rotor rings. In many instances, the bond, or the ring material underlying the bonded areas, fails which can lead to a potentially disastrous failure of the whole ring assembly.
Additionally, the relatively large differential expansion between the rotor rings requires the use of relatively soft elastomeric materials. Material softness, or excessive flexibility of the spacer rings, however, can give rise to a dynamic instability at high rates of rotation which induces vibrations. The latter in turn can damage the rotor, the rotor shaft, or the bearings in which the shaft is journaled. Increasing the hardness or stiffness of the spacer rings, on the other hand, limits the amount of relative dilation between the inner and the outer rotor ring which can be accommodated by the spacer ring. Consequently, prior art rotor rings must be either relatively thin, which renders the rotor wheel assembly more expensive, or that the rotor must be operated at a lower speed, because the spacer ring would otherwise fail due to its inability to accommodate the large relative ring dilations.
Thus, it is apparent that prior art multiple ring inertial energy storage rotor wheels have serious drawbacks which limit their potential use and which correspondingly limit the development of the otherwise highly desirable inertial energy storage wheels.